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No Drawing. Application filed December 28, 1927, Serial Re. 242, and in Germany November 17, 19 2 5.-

This invention relates to the hardening of iron or the like with the aid of a gas gen- 'erated in the hardening furnaces.

' .For thehardening of iron and the like it 5 has already been proposed to surround the .material to be hardened with a hardening powder and heat itin' a hardening furnace. It wasnecessary'in'this process that the article to be hardened should remain in intim mate contact at all points to be hardened with the hardening powder as otherwise the wellknown soft spots were formed. This process had several disadvantages? To begin with the carbon is set free too slowly so that the hardening process requires a relativel long time. Attempts have been made to s orten the duration of the process by employing a higher temperature than that normally selected (about 700 to 1000? C.) but this was found to beunsuitable as the higher temperatures render theharde'ned layer brittle and frequently even cause it to scale. A further disadvantage of this process consists in the uneven'heating of the article to be hardened,

26 for as the heating of a muflie furnace cannotbe so regulated as to progress evenly at all parts the articleto be hardened was frequent "ly heated first on one or another side whilst yet another side or the centre remained at a 3'0 cooler tem erature for a considerable time,

consequen y the hardening powder can only operate for varying perio s of time at va portions ofthe article and consequent y- 'anuuneven hardening of one and the same article results. Yet another disadvantageof this process resides in the fact that the depth ofhardening can only be inaccurately deteroi hardening powder (about 20 to 30% by lace at -a. uniform speed.

weight of the article to behardened) is re v quired as Well as a considerable period of time (2 to 5 hours) which means a consi'derable amount qf fuel.

It has also'been pro osed to harden iron and the like more particularly mass production articles such as screws, balls and the like, in rotating carbonizing drums. These furnacesconsist of heated rotatable drums into which the articles are inserted and into which ammonia which has previously assed over benzene, benzine or gasolene, isle from a closed device shown as the carburator. This-process also has disadvantages. On the one hand graphite isseparated from the ammonia benzene mixture at high tem ratures. carbon for hardening purposes. Further it adheres to the article to be hardened in the form of flakes, particularly as during the rotation of the drum individual iron articles fall over each other and thus press particles .This material 'is useless as com ined of graphite more firmly on to the iron which prevents the as acting on the graphite covered placesj hislikewi'se leads to the'wellknown soft spots; A further disadvantage of the process consists in the fact that t e hardening depends to a considerable extent on thequality of the benzene, benzine or gasolene employed. 'Thus a good hardening eflect'can only be achieved with a benzene of a'specific gravity of 06 1:00.62... If the.

benzene has a specific gravity'of 0.63 then the hardening is defective and the hard ening efiect ceases altogether if the" specific clear that this prevents a continuous uniform hardening as it is difiicult always to'obtain benzene of a quite definite specific gravity. A- third disadvantageof this prgcess resides in the necessity of employing the'carburator avity of the benzene reaches 0.7. It is which is a not altogether siniple. apparatiis and by the need bf operating with ammoniav 1 3 and benzene. The carbonizing drums which are in industrial use frequently. allow ampowder placed in the furnace and which completely decomposes therein. It has been found suitable to employ a hardening powder consisting of a mixture of substances which on being heated set free carbon and carbon dioxide whichact on the article to be hardened. It has also been found advantageous to add to the powder a nitrogen containing component as this expedites the hardening. A suitable composition consists for example of 50% lignite coke (a coke obtained from lignite or brown coal and in the German industry usually called Grude Koks) or some other low temperature coke, 30% barium carbonate (or some other carbonate which liberates carbon dioxide on being heated, such for example as calcium,

strontium, or magnesium carbonate) and 20% of ammonia (or an ammonium salt, such as amonium carbonate, ammonium chloride or a material which sets free ammonia on being heated such for example as urea, calcium-cyanamide, cyanamide, or dicyandiamide). It is probable that gaseous carbon compounds are formed or else that the coke favours the generation of carbon dioxide.

By way of an example about 5% by weight of the gasifying powder calculated on the weight of the charge to be hardened is placed in the drum together with the charge. The drum is then hermetically closed and heated under constant rotation. The. powder is quickly gasified and produces a pressure within the drum which has a favourable effect on the hardening process. This pressure can be adjusted according to the desired depth of hardening by accurately adjusting the composition and quantity of the powder employed. Thus, if more than 5% by weight of powder is used, the pressure, and there fore the depth of the hardening, is increased. The hardening takes place at between 720 and 820 C. and requires a time of from 15 minutes for a very small hardening depth to .1 1 hour for a hardening depth of 1 mm. in

the case given above in which 5% by weight of powder is employed. The advantages of the process reside in the absolutely uniform and previously calculable nature of the hardening, in the cheapness of the gasifying powder, which is only used in small quantities, in

the shortness. of the time required for the.

$5 hardening and in the absence of the carburator as well as of any trouble to the working due to gases. 7

Another suitable hardening substance consists of leather charcoal, barium carbonate and ammonia which completely'gasifies in the drum and effects the hardening simply by means of the gases evolved and therefore, as opposed to the packing system, the powder is used in quite small quantities, small accurately measured quantities however, according to the size of the drum, the quantity of material to be hardened, and the depth of the desired hardening, the drum being then closed and heated. The powder, then completely gasified, sets free in the drum a hardening gas of accurately calculable composition and at accurately calculable pressure. If, for'example, a number of charges of identical toothed wheels is to be hardened, and it is desired to produce an entirely identical hardening, then for each single charge, which naturally must be hardened in the same or an identical drum, a hardening powder of definite composition is employed, and is added in exactly the same quantities to each single charge. If now each charge is kept in the drum for the same length of time, then all the toothed wheels, hardened in the different drums will be of exactly the same degree of hardness.

In particular it has been found to be of advantage in iew of the various large drums and charges employed as well as for the particular kind of hardening desired, to form the quantities of powder to be measured into briquettes and to add the hardening powder of each charge in the form of one or more briquettes. The briquetted hardening owder ensures a lengthy and therefore uniform gasification. of the hardening powder avoids a sudden evolution of gas, which would cause a nonuniformity in the hardening of the different charges, since on account of the suddenly occurring pressure, gas escapes from the drum through the excess pressure valve, and consequently less gas acts upon the iron. There is also the advantage that the briquetting of the hardening powder prevents any evolution of dust and an accurate charging is permissible, so that an almost automatically uniform cementation is ensured.

It will be understood from the consideration given above that the difficulties met when hardening bodies of any kind according to any of the elder processes, were mainly due to the fact that the composition or the quantity or the pressure of the hardening gases was not sufiiciently under control, and that every variation of the gases effected the degree of hardening.

Itis further obvious that the process which has been described with reference to a carbonizing drum can also be employed in a fixed In particular, the briquetting furnace and the process shows exactly the same advantage in the latter case.

It is obvious that where a fixed furnace is employed the arrangement of the charge to be hardened must be such that those portions which are to be hardened are not prevented from being acted on by the gases due to contact with the furnace. This may be effected for example by suspending the articles to be hardened by means of wires, or hooks or sus pending them on other suitable supports, as. will be at once apparent to those skilled in the art.

I claim:

1. A process of hardening metals to a predetermined depth comprising placing a load ina container, placing in the container at a point remote from the load a charge comprised of approximately 50% of a carbon substance, approximately 30% of a carbonate. of a metal of the group of metals comprising barium, calcium, strontium and magnesium and approximately 20% of a material which produces free ammonia'upon being heated, the proportion by weight of the charge to the load being extremely small, heating the said container, substantially completely vaporizing the charge while in the container and uniformly treating the entire surface 0 the load with the vapor.

2. A process of hardening metals to a predetermined depth comprising placing a load in a container, placing in the container at a point remote from the load a charge comprised of approximately 50% of, a carbon substance, approximately 30% of a carbonate of a metal of the group of metals comprising barium, calcium, strontium and magnesium and approximately 20% of a material which produces free ammonia upon being heated, the weight of the char being ap roximately 5% of that of the loa ,Cheating t e said container, substantially completely vaporizing the charge while in the container, and uniformly treating the entire surface of the load with the vapor.

3. A process of hardening metals to a presubstance, approximately 30% of a carbonate of a metal of the group comprising barium, calcium, strontium and magnesium, and approximately 20% of amaterial which produces free ammonia upon being heated, heating the said container, substantially completely vaporizing the charge while the container, and uniformly treating the entire surface of the load with a vapor at the temperature of approximately 720 C. to 820 C. for a period of from fifteen minutes to an hour.

In testimony whereof I aflix my signature. HEINRICH- LINDHORST.

determined depth, comprising placing a load in the container, placing in the container at a point remote form the load a charge comprised of approximately 50% of lignite coke, approximately 30% of carbonate of metals of the group comprising barium, calcium, strontium and magnesium, and approximately 20% of a material liberating free ammonia upon heating, heating the said container, substantially completely vaporizin the charge while in the container, and'uni ormly treating the entire surface of the load with the vapor.

4. A process of hardening metals to a redetermi-ned depth, comprising placing a oad in the container, inserting in the container at a point remote from the load a charge com-' prised of approximately 50% of a carbon 

